This paucity of growth transactions is due to two main factors: (1) venture-backed companies are staying private longer due to the significant availability of private capital available to these companies; and (2) public companies are hamstrung due to activist investors, which are demanding actions like stock buybacks, dividend distributions and breakups - short-term actions to books stock price - at the expense of actions that enhance long term shareholder value such as investing in R&D or growth-oriented acquisitions.
It's a thought-provoking piece that's worth a read.
Link to blog post: http://a16z.com/2015/04/10/whats-holding-tech-ma-back/